Systemic lupus erythematosus (SLE) affects up to 1.5 million persons in the US. Although excess mortality has decreased in SLE patients since the 1970's, substantial morbidity persists. Fatigue is the most disabling and enduring complaint in patients affected with this chronic incurable inflammatory autoimmune disease. The ramifications of fatigue are significant and include decreased quality of life, an increased risk of work disability, and an associated increase in health care costs. The overarching goal of this R21 application is to explore ways to improve the measurement of two constructs, physical activity and fatigue, as a necessary first step in a broader effort to use a behavior management intervention to lower fatigue scores by increasing lifestyle physical activity in persons with SLE. We propose a novel application of a relatively new technology to objectively measure physical activity (triaxial accelerometry, which provides a validated measurement of daily physical activity in the community dwelling setting). In addition, we propose to measure covariates of fatigue using computerized adaptive tests (CATs) patient-reported outcomes (PROs) that have been developed using state-of-the-art cognitive, qualitative, quantitative and health survey methodologies as part of the NIH-funded Patient-Reported Outcomes Measurement Information System (PROMIS). PROMIS tools were developed to standardize measurement of self-reported health domains affected by many chronic illnesses and these tools offer the advantages of minimizing patient burden and maximizing precision. Using a cross-sectional study design, the following specific aims will be tested in this R21 application: Aim 1. Evaluate the frequency, intensity, and duration of physical activity as measured by accelerometry to obtain patient-specific average daily activity counts, average daily moderate-to- vigorous minutes of activity (MVPA) (defined as e 2020 activity counts/min), average daily minutes of light physical activity (< 2020 activity counts/min), and average daily minutes of any activity (i.e., minutes of non-zero activity counts) in patients with SLE. Aim 2. Characterize the cross-sectional relationships between objectively measured physical activity and fatigue (primary outcome, Fatigue Severity Score, (FSS) in patients with SLE. We will do this with and without adjustment for the major factors that influence fatigue including sleep and wake disturbance, depression, anxiety, and pain interference using PROMIS tools, and SLE disease activity and severity. The adverse clinical, social, and economic implications of fatigue support the critical need for an improved understanding of factors contributing to fatigue in SLE, a research priority identified in the NIH/NIAMS monograph entitled, The Future Directions of Lupus Research.